This invention relates to image projection systems and, more particularly, to an improved laser scanner system for an image projector useful for large screen display of electronic video information, such as high definition television.
A known scanning scene projector, in which the scene is painted onto the image plane using a high intensity light source, is the Scophony scanner developed at the Scophony Laboratories outside London during the 1930's, wherein an amplitude modulated video signal is modulated onto a collimated beam of light in an acousto-optic cell; the collimated beam fills a large portion of the acousto-optic aperture. The acousto-optic cell produces Bragg diffraction from the phase grating caused by spatially periodic variations of refractive index. By spreading the beam across the acousto-optic aperture, several "points" of the image are simultaneously modulated. This fan of information is scanned in the horizontal direction and imaged in the real image plane; because the image moves at the speed of sound in the acousto-optic cell, in order to view the scene an offsetting motion is provided to make the information stationary in the image plane. This is achieved in the Scophony system by a lens following the acousto-optic cell which produces a fourier transform plane at the plane of a multifaceted spinner rotating in the opposite direction so that even if the acoustic image is moving at the speed of sound, the offsetting motions freeze the image in space.
While the Scophony type of scanner, using a gas laser as the source of light, has adequate resolution for high definition projectors and has been produced commercially in the United States, Japan and Great Britain, it has not enjoyed wide acceptance for two reasons: (1) the low electrical-to-optical efficiency and high cost of gas lasers, and (2) the complexity and high cost of manufacturing and assembling the rotating multifaceted polygonal spinner.
Accordingly, a primary object of the present invention is to provide an improved image projection system.
A more specific object of the invention is to provide a laser scanner for an image projection system which does not require the use of a multifaceted polygonal spinner for stabilizing the image.
Another object is to provide, at a cost lower than for prior art scanners, a scanner capable of producing colored high resolution images.